Phase Portrait

Personal weblog of Ted Pavlic. Includes lots of MATLAB and LaTeX (computer typesetting) tips along with commentary on all things engineering and some things not. An endless effort to keep it on the simplex.

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Tuesday, March 22, 2016

At 8:00 AM this morning, I received a phone call from 302-279-3069 that I let go to voicemail. On the voicemail, there was a threatening but obviously scripted monologue along the lines of...

I'm calling regarding an enforcement action executed by the US Treasury requiring your serious attention. Ignoring this will be an intentional attempt to avoid initial appearance before a magistrate judge or Grand Jury for a federal criminal offense. My number is 302-279-3069. I repeat 302-279-3069. I advise you to cooperate with us and help us to help you. Thank you.

So what do you do? If you have an Android device (in principle, you could use an Android emulator as well), you can download the NOOK e-reader Android app and, through a few steps, get the same ePubs you used to download. Here's how you do it:

Open the NOOK e-reader app and find your library with your newly purchased book

Select "Download" to cache the book locally on the device, and wait for the download to finish

Once the download is finished, open a file manager application like ES File Explorer and navigate to/sdcard/Nook/contentYou should find your NOOK book ePubs in that folder. That is, find the Nook folder in your internal storage, and then find the content folder inside it.

You can use your file manager to copy those ePubs out of that folder, or you can "Share" them to another installed app like Dropbox or Google Drive so that you can easily get them on other devices or your Desktop computer

Once you have the ePubs on another device, you can do whatever you used to do with them when you could download them directly from the NOOK library on-line in your web browser.

[ Side note: You might try to plug your Android device into your computer and use MTP to access /sdcard/Nook/content directly from your Desktop and copy files out of it. However, it's possible that the ePubs won't be accessible this way. In fact, the might not even show up in the file listing. So you might be stuck using a file manager on your device. ]

Tuesday, January 13, 2015

I recently signed up for the Dollar Shave Club (DSC), which is a site that lets you paying $6 every 1 or 2 months for 4 razors by mail (and you can pause your subscription at any time). It turns out that you can get the exact same razors for much cheaper with no monthly subscription so long as you're willing to buy in bulk. Apparently DSC gets its razors from Dorco USA, which you can buy directly from on-line. In fact, you can buy retail handles with trial razors from Dorco too. For example, for $6, you can try out a Dorco handle and two razors, and then you can buy 36 replacement cartridges for less than $1/razor.

I tried my "4X" DSC (or Dorco USA) razor for the first time this weekend. It was an OK razor. It had a nice heavy handle, but the angle of the pivoting head was not as steep as I'd like, and the moisturizing strip was far too thick. Consequently, it was very awkward to get into small nooks -- like under the nose. The "4X" razor doesn't have a trimming edge either (the more expensive 6-blade "Executive" model does), which would really improve matters. But after the shave, my face felt pretty good. It just took a lot longer to get through a full shave. So I'm not sure I can fully recommend these razors over, say, Schick Quattro Titanium disposable razors, which actually have a trimming edge and are pretty cheap from local stores or from Amazon (around $7/3 razors).

DSC also sent me a tester of their "Dr. Carver's Easy Shave Butter," which actually burned my face a bit as it went on (but I have sensitive skin). My face was fine afterward. But using Nivea For Men Active3 Body Wash for Body, Hair & Shave worked just as well without the sting. So I'm not sure I can recommend the Shave Butter either.

So I'm torn. It's possible that the 6-blade+edging "Executive" unit will be better. Either way, in the long run, it's cheaper to buy from Dorco USA directly, and I like the idea of dropping the subscription service. Dorco USA also sells other razor models and even disposable models. They also sell razors designed for women, whereas DSC just tells women to buy the 4X. The 4X has a handle which is somewhere between what you'd want for the face and the leg, but I think it's biased toward face shaving. So the lady's models at Dorco USA are definitely recommended if you need to shave legs.

So that's my brief review. tl;dr -- Check out DORCO if you like DSC's blades but don't like the subscription service.

Monday, December 29, 2014

My father needed a portable Bluetooth-capable MP3 player with large or expandable memory. Portable MP3 players with such features typically sell for $80+ (Trio mini), with the nicest units being at least $150 (iPod nano) or much more. The cheap models have terrible battery life (3 hours for Trio mini). Creative solution? He bought a no-contract LG Optimus Exceed 2 from Verizon Wireless for less than $50 at Best Buy and used a 16GB microSD card that he already owned.

This particular no-contract phone is fast, runs Android 4.4.2, and can be used without activating. In fact, it is very easy to bypass the initial activation screen with a special key sequence – volume up, volume down, back, home – pressed after restart at the language selection screen. After bypassing that screen just once, the phone is happy to run unactivated indefinitely after that – it will never present that activate screen again. Use airplane mode to prevent phone from looking for a network to save battery, and flip on WiFi and Bluetooth. It's like a $50 iPod touch. Nice deal.

You can get this phone from a number of places, as shown in links below. Of course, there are other no-contract phone choices you could make (check all no-contract carriers – Boost Mobile, Virgin Mobile USA, T-Mobile, etc.). If you're willing to compromise on the phone software or other capabilities, you can get decent "MP3 player phones" for $30 or even $20.

Monday, February 04, 2013

On large multi-university NSF grant proposals, NSF requires that senior personnel submit a 2-page biographical sketch ("biosketch") that is formatting according to certain rules in their Grant Proposal Guide (GPG). The format is pretty simple, and so there does not seem to be much demand for a solid LaTeX template for one. Nevertheless, I thought some people might find one helpful.
I've posted a PDF of my NSF-style biosketch below along with the TeX source used to generate it.

Thursday, January 24, 2013

I was contacted recently by e-mail asking how to produce a phase portrait of a discrete-time system. In my initial response, I explained that a true "phase portrait" wasn't defined for discrete-time systems because the technical notion of a phase portrait depends on a special structure that comes along with ordinary differential equations. The original poster needed some additional clarification, and so I sent a second e-mail that I have posted below. It touches a little bit on the original poster's question, it comments on differences between discrete-time and continuous-time systems, it talks a bit about chaos, and it gives a brief description of Poincaré/return maps that are often used in the study of approximately periodic systems.

My point is that a discrete-time system really cannot be interpreted within a field-theoretic framework. A "phase portrait" captures both position and momentum of a continuous-time system described by an ordinary differential equation. These momentum variables setup the "field" that gives structure to the phase portrait. In a discrete-time system, we don't have the same kind of momentum.

For a continuous-time system, I can plot a point at an individual position, and I can also then draw a vector pointing away from that point representing the velocity at that instant of time. It is these velocity vectors that are put together to make a phase portrait of the system.

For a discrete-time system, there is no point-based velocity. In order for me to calculate the approximate "velocity" at a point, I need to know the position at the next point. Then I can draw a line between those two points and approximate the "velocity" of the system as going from the first point to the second point. However, if I have to know the next point in future anyway, it's more useful to just draw the second point.

Now, some discrete-time systems have a more predictable structure. For example, if you have a linear time-invariant discrete-time system like:

x[k+1] = M*x[k]

then the algebraic structure of the "M" matrix gives us insight into how trajectories will evolve. So for these special cases, it is possible to draw a kind of "phase portrait" for the discrete-time system. However, this is primarily because such a discrete-time system can be viewed as a sampled version of a continuous-time linear time-invariant system which does have a phase portrait.

So, for an arbitrary discrete-time system, the best thing you can do is explore trajectories from different initial conditions. Gradually, as you explore the space more and more, you may find boundaries of attractors (possibly strange attractors). A complication with discrete-time systems is that the "next" point may be very far from the "previous" point. Take, for example:

x[k+1] = -1.1*x[k]

If you start at x[0]=1, the trajectory will bounce from point clustered above 1 to points clustered below -1. In a continuous-time system, you might expect to see initial conditions above 1 stay near 1 and initial conditions below -1 stay near -1. That is, in a continuous-time system, you wouldn't imagine trajectories could cross x=0 (which is an equilibrium/fixed-point of this system). However, the discrete-time system can jump wildly from point to point.

Take, for example, the Henon map you mention. Wolfram's Mathworld has some nice plots:

The first pair of side-by-side plots are colored "according to the number of iterations required to escape". That is, the plots were generated by starting at several initial conditions and recording the resulting trajectories. Each "iteration" gives the next point from the previous point. For a while, a trajectory will stay around its initial condition. Eventually, it will escape and move away from the region. The regions are colored based on how many iterations (i.e., how many calculations after the initial condition) it took for the trajectory to leave the region.

The second pair of side-by-side plots show a SINGLE trajectory started at x[0]=0 and y[0]=0. Each point was recorded and gradually a pattern emerged. Notice how in the left plot the two regions appear to be disconnected. If you saw a phase portrait that looked like this in a continuous-time system, you would conclude that initial conditions within one region would not be able to join the other region for this set of parameters. However, this plot was generated from a SINGLE initial condition. So the plot jumps from points in the top left to points in the bottom right and back.

So that's how you can explore something like the "phase space" for discrete-time systems. You can probe it with different initial conditions. For chaotic systems, you have to be very careful you don't accidentally jump over an interesting region of initial conditions that may have qualitatively different trajectories that follow from them.

As an aside, I guess it's also worth mentioning that many popular discrete-time chaotic maps are actually Poincaré maps of continuous-time dynamical systems. Poincaré maps have other names, including "return maps." Consider, for example, the planets as they orbit the sun. The actual orbits of the planets in three dimensions looks like a tangled mess when you consider their histories over several cycles around the sun because each orbit is slightly different than the previous orbit (i.e., they aren't entirely planar). However, if you insert a plane perpendicular to their orbits at a single location, each planet pierces the plane at one point every cycle. The resulting shapes that are poked out of that cross section reveal structure in the orbits.

Tuesday, October 09, 2012

For some reason, the ACM has baked in chicago-like citation macros into the ACM SIG proceedings LaTeX templates instead of using the far superior natbib that literally everyone else on the planet uses.
I'm much more accustomed to typing \citet as opposed to \shortciteN, \citeauthor as opposed to \shortciteANP, etc. So I decided to add this little translation table to my preamble:

While on that page, click on the down arrow in the icon in your your Firefox search toolbar. You should see several "Add" options at the bottom. That lets you add several different types of CTAN search to your toolbar.
You can then "Manage Search Engines" and add a keyword to some of them. I use the keyword "ctan" to search package ID's and the keyword "ctanp" to search over package ID's and descriptions. The keyword lets me search directly from the awesome bar without having to use the search bar.
[ These definitions were inspired by firefox_ctan_plugins by Martin Engler, but those don't work with CTAN anymore. ]

Sunday, July 29, 2012

This message goes out to ASU faculty, staff, and students. Doesn't it make you mad when you're off campus and you want to read a paper and the research database that holds the paper doesn't let you view it because you're not a subscriber? I hate that.

Luckily, the ASU library system provides an off-campus scholarly portal from my.asu.edu (under the "Library" option on the left) that can be used to make your off-campus connection look like an on-campus one so research databases will grant you ASU-caliber access. Unfortunately, if you've clicked on a link to a paper, you have to re-do your search within their portal to get your off-campus access, and that's really inconvenient. So I've come up with a shortcut (similar to my OSU LOCSI shortcut).

Drag the following bookmarklets to your "bookmarks toolbar" (that strip of bookmarks that rides just below the "location bar" in your browser).

When you're on a page you'd like special access to (e.g., the official site of an academic paper of interest to you), click on the bookmarklet and you'll be transported to that site via ASU's off-campus sign in. You may have to use your ASU username and password to login the first time you click on the bookmarklet, but after that you should be granted ASU-level access quickly.

(feel free to rename that bookmarklet as you wish; the name "ASU LOCSI" isn't important to their function)

Tuesday, June 19, 2012

I know someone who recently purchased an Asus Transformer TF300T from MicroCenter in the store for a cool $350, which means that after tax it was still cheaper than it was selling for at Amazon ($384). Strangely, I can find no mention of the TF300T on MicroCenter's website (maybe local store was just clearing inventory and they don't plan on selling them in the long term?). Anyway, this is the first Android tablet this person has ever used, and they were interested in getting the ability to print from it to the printers in their home. Here's how I responded (text copied from an e-mail and then marked up a bit).

Here are some tablet printing options that I've tried that appear to still be popular. I've put some footnotes at the bottom of this message that are tangential topics that may still be interesting to you. For example, the first footnote [*] is about a way to pick printers in the future that allow for cloud printing without the aid of a PC being on.

The first two apps I currently have installed on my phone and am happy with. You would probably only need one of them. The third app is one I tried, but I gave up on because I thought the first two apps were just as good or better. I have not used the fourth app, but you should know it exists as it would be handy if on the go and you need a hard copy of something.

First, here's "Cloud Print," which is a free app with no limitations but will display ads unless you donate to it:

Note that if Chrome is already installed on another computer in the house, and if that person has shared her printers with you, you should already be able to print to printers connected to her computer even if your computer isn't on (but hers has to be on). Once printers are connected to your Google account's Cloud Print, then you can use the "Share" button (it looks a little like a tree with two branches, typically) from any app to export whatever you're looking at to the "Cloud Print" app. That document will then get printed to the Cloud Print printer you choose. Note that "Print to PDF" is always available (and it will store that PDF on your Google Drive, I think, which you can access using the "Google Drive" app I just e-mailed you about). There are competing Android apps to Cloud Print, like "Easy Print:"

PrinterShare's features largely overlap with Cloud Print. That is, Cloud Print does a couple of things PrinterShare doesn't, and PrinterShare does a couple of things that Cloud Print doesn't. For example, you can use Google Cloud Print with PrinterShare, and so any printer that you can use with Cloud Print, you can also use with PrinterShare. One major difference is that PrinterShare also supports WiFi printing to network-enabled printers (e.g., the HP LaserJet 2055dn) and some print servers. Initially, the support was spotty. However, I'm noticing more and more printers start to pop up on my WiFi list when I check. It's a solid app, but it's a little annoying that if you switch printers frequently, it has to re-download print drivers nearly every time. Downloading print drivers is very fast, and it's not much of an annoyance, but it's a little confusing why it can't keep these installed. Otherwise, it's a solid application that's a tough competitor, which is why it maintains such a high price for it's premium key.

I have also used the "PrintBot" app, which allows for 3 printouts a month unless you pay $4.50 for the full version (which is unlimited). It supports printing directly to a print server (no Google Cloud Print required), but it is not as easy to setup as PrinterShare. So unless they've improved things a bit since I used it last, I don't recommend it. In theory, it may be the only tool that can print to the print server connected to the HP LaserJet 5 upstairs without using Google Cloud Print; however, PrinterShare may have improved to be able to do that natively anyway:

that allow you to print to public printing locations, like FedEx Office stores, UPS Office stores, Walmart photo kiosks, hotels, and others. This is a neat idea if you're on the go and need a paper copy of something. The downside is that although the app is free, the place that prints your document may charge you per page.

In summary, if you're OK with having a computer on and connecting your legacy printers to the Google "cloud print" service, then give the first free app ("Cloud Print") a shot. If you want the ability to print over WiFi directly without computers being on, then consider the second app ("PrinterShare", which is pretty expensive for an app if you want to print more than 3 things a month). Note that the WiFi option only works if you're on the same network as the printer. You cannot print from remote with PrinterShare unless you're using Google's cloud print service (and thus have a PC turned on).

--Ted

Footnotes:

[*]: If you find yourself buying a new printer in the near future (not likely), you can choose a "Cloud Ready" printer:

These printers connect to servers at HP, Kodak, Epson, or Canon that allow access to them from remote provided the correct username and password. Google Cloud print can print directly to these without the aid of a computer being on.

In general, any network-enabled printer will probably have good Android support even if it's not "cloud ready." In fact, some vendors release their own apps to make it simpler to print to them from Android, like Lexmark (but I think generic network printer apps will connect to these too, in most cases):

[**]: If you like Google Chrome as a web browser, you can use it instead of Firefox. If you have Firefox bookmarks, Chrome will import them. Also (and this might be interesting to you), you can share your Chrome bookmarks and tabs to your tablet and so you can easily go from one to the other. If you don't like Chrome but like the idea of getting your same bookmarks and tabs on your tablet as is on your Desktop, that's possible with Firefox too. Either way, we should probably install the "Chrome to Phone" app